It is now well established that there is a worldwide energy shortage and particular emphasis has been placed upon new technologies for the conversion of our naturally abundant resources such as coal to more broadly useful gaseous materials such as syngas (CO/H2), hydrogen (H2), methane (CH4) and other low molecular weight hydrocarbons. Additionally, the economical production of natural gas and high-value energy sources that can be used in place of natural gas has become an economic necessity. Natural gas has a composition that includes about 80-85 percent methane (CH4), about 10 percent ethane (C2H6) with varying percentages of higher hydrocarbons including propane (C3H8) and butane (C4H10). The primary component of natural gas, methane, has a heating value of about 23,875 Btu/lb.
There are particular advantages to natural gas and other high BTU gas compositions being produced on site and utilized in such energy consuming applications as the production of electricity. Although the combustion of coal to generate steam, can be used to generate electricity, coal deposits frequently contain high levels of impurities such as sulfur and mercury and consequently must be subjected to expensive processing prior to utilization. Thus, the direct conversion of the carbon in the coal to a hydrocarbon product of high purity, such as methane gas, remains a highly desirable objective. Many processes currently available have not proven to be economic. New processes that are both highly economical and ecologically sound must be developed to satisfy current energy and raw material demands that can potentially use low-grade coal sources.
In marked contrast to coal gasification, incineration typically involves a combination of pyrolysis (1200° C.) and combustion that is initiated by a high temperature flame. At such high temperatures, pyrolysis transforms organic compounds into a more oxidizable form, but the oxidation step requires the actual collision of the resulting incipient high-energy carbonaceous fragments with oxygen. Lack of efficient mixing on a molecular level impedes the rate of oxidation. Alternatively, more modern methods of coal gasification are initiated at high temperatures but the coal is injected into hot molten metal baths for the purpose of thermal decomposition into molecular fragments followed by certain other chemical transformations of the atomic carbon into usable gaseous species.
In Rummel's U.S. Pat. No. 2,647,045, for example, a molten slag bath obtained from the reduction of iron ore or from the “non-combustible residues of coal products” is circulated and finally divided coal is injected into the bath and a separate addition of air is also conducted along with “an endothermic gaseous reactant”, e.g. water and CO2. U.S. Pat. No. 3,700,584 by Johanson treats low quality coal having a high oxygen content in a gasification process converting the coal to a useful hydrocarbon product.
An iron bath is used for coal gasification in U.S. Pat. No. 4,388,084. In U.S. Pat. No. 4,389,246 issued to Okamura et al., on the subject of coal gasification employing a molten iron bath, the bottom-blowing of ethane is described. The ethane or other hydrocarbon gas is used to stir the mixture and is considered by Okamura et. al. to be equivalent to oxidizing gases. Injection from above is employed in Gernhardt et. al., U.S. Pat. No. 4,043,766; Okamura et. al. U.S. Pat. No. 4,389,246: Okane et. al. U.S. Pat. No. 4,388,084; and Bell et. al. U.S. Pat. No. 4,431,612.
Other processes for producing synthesis gas from steam and carbon have been disclosed. For example, U.S. Pat. No. 1,592,861 by Leonarz describes a method for the production of water gas by contacting steam with uncombined carbon in a bath of molten metal. The steam is dissociated into its respective elements at temperatures of 900° C. to 1200° C. The carbon combined with the oxygen of the gas is sufficient in quantity to produce carbon monoxide but not to make an appreciable quantity of carbon dioxide.
Molten iron is employed by Rasor in U.S. Pat. Nos. 4,187,672 and 4,244,180 as a solvent for carbon generated through the topside introduction of coal; the carbon is then partially oxidized by iron oxide during a long residence time and partially through the introduction of oxygen from above. For example, raw coal can be gasified in a molten metal bath such as molten iron at temperatures of 1200° C. to 1700° C. Steam is injected to react with the carbon endothermically and moderate the reaction. The Rasor disclosure maintains distinct carbonization and oxidation chambers.
Bach et.al. in U.S. Pat. Nos. 4,574,714 and 4,602,574 describe a process for the destruction of organic wastes by injecting them, together with oxygen, into a metal or slag bath such as is utilized in a steelmaking facility. Nagel et.al. in U.S. Pat. Nos. 5,322,547 and 5,358,549 describe directing an organic waste into a molten metal bath, including a first reducing agent which chemically reduces a metal of the metal-containing component to form a dissolved intermediate. A second reducing agent is directed into the molten metal bath. The second reducing agent, under the operations of the molten metal bath, chemically reduces the metal of the dissolved intermediate, thereby, indirectly chemically reducing the metal component of the waste.
Hydrogen gas (H2) can be produced from feedstocks such as natural gas, biomass and water (steam) using a number of different technique. U.S. Pat. No. 4,388,084 by Okane et al. discloses a process for the gasification of coal by injecting coal, oxygen and steam onto molten iron at a temperature of about 1500° C. The manufacture of hydrogen by the reduction of steam using an oxidizable metal species is also known. For example, U.S. Pat. No. 4,343,624 by Belke et. al., discloses a three-stage hydrogen production method and apparatus utilizing a steam oxidation process. U.S. Pat. No. 5,645,615 by Malone et al. discloses a method for decomposing carbon and hydrogen containing feeds, such as coal, by injecting the feed into a molten metal using a submerged lance. Malone et. al. in U.S. Pat. No. 6,110,239 describe a hydrocarbon gasification process producing hydrogen-rich and carbon monoxide-rich gas streams operating at pressures above 5 atmospheres where the molten metal is transferred to different zones within the same reactor effected by vertical baffles for the purpose of modulating carbon concentrations.
The method of Kindig et.al. in U.S. Pat. Nos. 6,682,714; 6,685,754 and 6,663,681 describes the production of hydrogen gas, formed by steam reduction using a metal/metal oxide couple to separate oxygen from H2O. The method utilizes one of the fundamental operations of Nagel et.al., U.S. Pat. No. 5,358,549 that reduces a dissolved metal oxide with a carbon source such as CO or coal. Steam is contacted with a molten metal mixture including a first reactive metal such as iron dissolved in a diluent metal such as tin. The reactive metal oxidizes to a metal oxide, forming a hydrogen gas and the metal oxide can then be reduced back to the metal for further production of H2 without substantial movement of the metal or metal oxide to a second reactor. It is suggested that preventing the physical movement of such nongaseous materials such as Fe and FeO, on a commercially useful scale involving several hundred tons of material, may reduce the cost associated with the production of hydrogen gas. Kindig et.al. (U.S. Pat. No. 7,335,320) produces a hydrogen-containing synthesis gas (H2:CO) in a 1:1 molar ratio using several hundred tons of material without the need to remove carbon oxides from the gas stream.
A number of metal/metal oxide systems have been used in addition to iron/iron oxide. For example, U.S. Pat. No. 3,821,362 by Spacil illustrates the use of Sn/SnO2 to form hydrogen. Molten tin is atomized and contacted with steam to form SnO2 and H2; the SnO2 is reduced back to liquid tin.
Davis et.al. in U. S. Publication. No. 20060228294 (European Patent EP1874453) have described a process and apparatus for treating organic and inorganic waste materials in a high temperature metal bath reactor to produce syngas by the reaction of steam with iron metal. However, this process requires injecting oxygen, steam and/or co-feeding one or more additional feed materials of higher heat value in order to maintain a balanced production of syngas.
In spite of the above mentioned processes, however, a more efficient method of producing syngas at high pressure and high temperature remains a desirable goal.